1. Field of the Invention
The invention relates generally to interconnection systems for use in electrical and electronic connectors, including two-piece, card edge, and wire interconnections. In particular, this invention relates to an improvement in fine pitch connectors for connecting printed circuit boards (PCB) for applications including board stacking, vertical to vertical, mother to daughter, vertical to right angle and/or straddle, and in one aspect relates to an improved connector comprising a plug and a socket each having four rows of electrical contact elements.
2. Description of the Prior Art
The art is replete with connectors for making multiple interconnections between boards, between boards and discreet wires, and between boards and flexible circuits, all of which have the goal of making the most interconnections per area of board space.
For example, board to board connectors are illustrated in PCT Application WO 93/03513 published Feb. 18, 1993 and in U.S. Pat. No. 5,380,225 issued Jan. 10, 1995. The publication illustrates a board to board interconnection of the hermaphrodicitic design wherein the connector portions have the identical shape and are mated in a single orientation to ensure proper electrical connection. Further, the solder tails of the connector portions are spaced 1 mm and each portion of the connector is formed to have a row of passive contacts (fixed contact surfaces) and a row of active contacts (movable spring contract surface). This relationship, according to the publication, reduces the required overall PCB to PCB stack height (the distance between two coupled circuit boards) because only one spring height is required. Further, since each connector has both spring contacts and fixed contacts, the spring force on the movable contacts is the same from its initial mate height until the final mate height. The movable spring contacts are deflected by the same predetermined amount regardless of the PCB to PCB stack height. The latter patent referenced above teaches the use of a connector making two rows of contacts, each row including staggered contacts. This connector however discloses the contact elements of a passive nature in the plug 1a and the active, flexible contacts in the jack 1. The contact elements are however all spaced and staggered to form the four rows of contacts of equal number in one connector, lengthwise thereof. Other PCB to PCB interconnections are shown in WO 90/16093 where opposed spring contacts were employed which increased the stack height.
U.S. Pat. No. 4,804,336 discloses a D-shaped connector having improved density by using staggered rows of pin contacts in the body to double the density from the normal 50 contacts to 100. As in U.S. Pat. No. 5,380,225, staggering and duplicity alone does not serve to adequately improve the density of the interconnections to be made and still reduce the stack height.
Historically, separable two-piece connectors are either of pin and socket style or ribbon style. Pin and socket connectors typically utilize a substantially straight, solid copper alloy pin of primarily round or square cross section with the tip of the pin shaped in one of many ways to provide alignment to and deflection of a mating contact. These pins are typically covered with a precious metal plating and are then installed in an injection molded housing to position and to electrically isolate each pin. They are often presented in two symmetrical rows of pins. Typically, distance between pins within a row and distance between rows of pins are equal. A socket contact can take on a wide variety of forms, but is usually contained inside a housing which receives the rows of straight pins with a shaped end feature. A socket contact is typically "active," meaning that physical changes of the dimensions, reaction forces, and internal stress levels in the contact material occur during mating with a pin. A pin contact is typically "passive," meaning that no changes, or very limited physical changes, occur during mating. One example of an active socket type is known as a "spring contact" due to the fact that it deflects during mating with a pin and reacts by providing a normal force against the pin. Spring contacts may also act to absorb variations in sizes of contacts, variations in positioning of contacts in a housing, and other variations that may occur during mating.
Ribbon based connectors typically utilize a substantially rectangular, copper alloy pin that is covered with precious metal. The ribbon systems differ from pin and sockets in that both contacts are usually rectangular in shape and each typically mates with a like contact in the flattest or longest dimension of the contact. In addition, these contacts are generally open and visible from the separable side of both connector housing halves of a mating system. Rectangular portions may also be configured on a board mount or cable mount side of a connector pin as well. Ribbon systems like pin and socket systems have in the past utilized one contact type in the socket housing and a different contact type in the plug housing. It has also been observed that some systems use the same type contact in both the plug and in the socket, but in a reverse orientation. A ribbon system may have active contacts in one housing and passive contacts in the other, or both housings may contain active contacts which mate with one another. Conventional ribbon systems have embodied two rows of contacts in a single connector housing with each row having the same number of contacts present.
A typical active (or "spring") contact has a cantilever beam design that includes a metal contact mounted in a connector housing constructed of a material such as plastic. In such a design, one end of the cantilevered spring contact is relatively free to move or deflect within the housing, while the other end of the contact is relatively fixed in the connector housing material. The point at which a contact is secured to a connector housing may be referred to as the "fixed point." When the connector housing is mated with a corresponding connector component, the free end of the cantilevered contact is deflected by contact with another contact element, such as a pin or a passive or active ribbon contact. The point where the two contact elements meet may be referred to as the "contact point." This deflection serves to induce internal stress in the active contact or contacts which, in turn, results in generation of a reaction force against the other contact. This reaction force is important, as it forces the contacts together at the contact point in such a way to enhance electrical contact and to reduce electrical resistance between the two contacts (known as "constriction resistance"). Reaction force is a function of the cross section of a contact (width and thickness), as well as its length. Most importantly, both internal stress and contact normal force are inversely proportional to distance from the contact anchoring point, or contact base.
Traditional cantilevered active spring contact designs suffer from several disadvantages. Internal stresses generated by deflection of an active spring of the cantilevered design typically diminish rapidly with distance from the base of the spring toward the end of the contact and/or the contact point. Because these internal stresses are fully utilized only at the base or fixed point of a contact, force present at the contact point is reduced as a function of distance from the contact base or fixed point, resulting in degraded electrical contact and increased constriction resistance. Constriction resistance may be a primary cause of heat generation when current flows through a connection. Heat generation in turn may cause stress relaxation in contact materials, resulting in a further decrease in contact normal force and a further increase in constriction resistance and heat generation. This may become a self-perpetuating process, in which additional heat is transferred to the surroundings and stress relaxation continues. This process may continue until a connection becomes open or until surrounding materials soften, melt, or burn.
Another disadvantage of the traditional cantilevered contact is the occurrence of plastic "creep" at the base of a deflected spring contact. As discussed above, maximum internal stresses are present at the fixed point where a deflected spring contact is anchored in a connector housing. Over time, reaction forces generated by a metal contact against a plastic housing typically causes the plastic to yield or "creep". This phenomenon may result in a shifting of the contact base and a resulting shift in the effective fixed point of the contact to a location below the original base of the contact. This phenomenon causes an increase in the effective deflection length of the contact and a corresponding reduction in the contact normal force generated by contact deflection. As described above, with decreased contact normal force may come increased contact resistance and operating temperature. Decreased contact normal force may also make the connection susceptible to shock and vibration disturbance from sources such as cooling fans and transportation motion. Finally, when deflected under stress, cantilever beam spring contacts are susceptible to permanent deflection and/or overstress. Permanent deflection of a spring contact may result in a reduction in internal stress and contact normal force. This may also contribute to an increase in constriction resistance.
Thus, a contact configuration capable of maintaining internal stress and contact normal force at a distance from the fixed point of a contact, and for an extended period of time is desirable.
U.S. Pat. No. 4,420,215 to Tengler discloses a cantilever contact configuration with a contact arm having an effective length that varies during deformation in response to a member inserted to engagement with a contacting means. The contact disclosed in Tengler has a curved or bowed shape that interacts with a linear surface of a connector housing. Among the disadvantages of the contact design disclosed in Tengler is an increased connector width required to house the profile of the shaped contact. This need for increased width is undesirable in view of the demand for increasingly miniaturized components.
An alternative approach to Tengler is shown in patent application DE 3703020, which shows a contact configuration in which a portion of a contact spring extending between a support point and a contact area is progressively shortened in the course of deflection of the contact area. In this case, the contact has a linear shape that interacts with a curved surface of a connector housing.
In addition to electrical connector contact problems, printed circuit boards which receive or engage connector products typically suffer from some degree of one dimensional bowing or two dimensional warpage/twist to them. These boards may also vary in thickness. Such nonuniformities may cause difficulties in connection configurations involving circuit boards. For example, when mounting a surface mount connector to a bowed or warped board, it may be difficult to obtain uniform and/or effective solder connections between connector compact tails and board solder pads. In addition, bowed or warped circuit boards may be difficult to align and/or insert into a card edge connector housing, decreasing the reliability of the connection. Also, connectors are generally being configured with increasing pin counts and as a result are being built longer even in the presence of higher densities. Increased connector lengths exacerbate the problem because printed circuit board bowing, warpage, and/or twisting typically worsen with increased connector length and width. Further, many connector users are migrating to more connector installations that utilize surface mount processes which do not have the benefit of long tails extending into and through holes in the board. Because surface mount configurations depend on contact between connector feet and surface pads as described above, bowing, warpage, and other variations in board surface characteristics may particularly impact connection integrity of longer, higher density surface mount connections. Finally, board attachment processes are utilizing higher and higher temperatures to fully activate solder paste to ensure that all joints are fully reflowed and these higher temperatures also increase board warpage. Because board warpage is typically caused by differences in coefficients of thermal expansion between different layers of a laminated circuit board, these higher temperatures also may increase board warpage, thereby exacerbating connection problems.
Typical card edge connector systems employ a connector housing with a cavity for receiving a card edge. A card edge typically employs a number of passive contacts and the connector housing typically contains a number of active contacts for mating with the passive contacts of the circuit board card edge. During mating of a card edge with a connector it is important that the board and connector housing contacts be aligned prior to engaging so that contacts are not damaged and proper connection is made between the two parts. In the past printed circuit boards have been provided with features, such as through holes for aligning connectors to a board. These through holes are typically engaged by latching features mounted on engagement members, such as cantilever spring or pivotally mounted moveable arms. Not only do these holes and latching members fail to provide alignment during mating of a card edge with a connector, but these mechanisms also latch a card within a connector housing by means of a force applied normal to the side of the card edge, which may tend to push a board to one side or the other of a connector housing potentially resulting in unbalanced forces being applied to the mated contacts. In addition, the cantilevered or pivotally mounted latching members may be bulky and difficult to construct. Thus, a mechanism to anchor a connector to a board despite such board nonuniformities is desirable.
In other cases, card edge connectors are constructed such that a polarization means, such as a rib, provides alignment to a slot routed in a printed circuit board. The mating portions of these connectors are typically rigid and fixed in position, therefore requiring that a clearance be provided between the polarization rib and the slot sidewalls in all conditions of feature size and placement in both parts, respectively. In addition, a typical circuit board slot feature is usually formed or placed on a printed circuit board in separate step and relative to the tooling holes. The conducting contact pads on the printed circuit board are also typically positioned in a separate step and relative to the same tooling holes. Because of the separate step, a number of tolerances and clearances are typically required in a conventional card edge connector system. These tolerances tend to be cumulative in nature, and therefore work against a fine pitch interconnection system for card edge configurations by producing mating components that result in conducting contacts which fail to, or only partially contact the border of a mating conductor pad. Furthermore, due to the additive nature of tolerances in the positioning of latching holes and contact elements on a circuit board card, these latching holes may not provide proper alignment of connector housing contacts with circuit board contacts when engaged with the latching member features. Consequently, a mechanism for properly aligning the contacts of a circuit board and mating card edge connector, and of anchoring the card edge and connector in this aligned position without exerting forces normal to the side of the circuit board is desirable.
Among other problems related to connector technology are those that arise when surface mounting a connector in a straddlemount configuration. In this configuration, conducting pads of a printed circuit board are typically positioned near the edge of the board and are usually present on both sides. When connecting a connector to a board, problems may develop in correctly positioning the conducting tails of contact elements in a lateral direction (i.e., sideways) with respect to printed circuit board edges, as well in a longitudinal direction (i.e., in and out of the board) in the direction of connector attachment.
Typically, a mechanical fastener is presented and affixed to each end of a straddlemount connector before or after solder reflow, typically performed by hot bar or by heating solder paste. Presenting mechanical fasteners in either condition increases the cost of the placement operation. There is also a cost associated with possible damage done during the assembly. In addition, typical designs of this nature rely on conducting contact tails to hold a connector on the board during handling, during solder attachment processes, and during subsequent handling afterwards. It is likely that movement or misalignment will occur in these periods. This is especially true since the board often will be placed on a conveyor which travels through an oven. In this case, a straddlemount connector typically prevents the board from being laid flat on the conveyor and thus a twisting load or torque is placed on the connector. This creates an unbalanced force arrangement on the conducting contact tail portions. The net result is that the connector can be soldered in an incorrect position (e.g., tilt or off center), or that the conducting contact tails will be soldered more on one side than on the other side. Thus a straddlemount connecting device capable of fixing a connector to a printed circuit board in a simple manner and in a way which protects contact tails from movement or misalignment during handling or manufacture is desirable. In addition, a straddlemount connection mechanism that would provide alignment of the contact tails to circuit board solder pads is particularly desirable.
Conducting tail and board attachment portions of conductors in any connector product are important as once set, they heavily constrain the manufacturing processes of a connector and the manufacturing process for assembly of the connector to a printed circuit board.
Almost all products in the electronic industry are continuously being replaced by smaller and faster products. In the case of connectors, product sizes are primarily driven by the host product which the connectors serve. This means that the conducting members are smaller (shorter, thinner, and/or narrower) and are being positioned closer together. The reduction in size of the conductors enables faster electrical signals to pass through the connector. However, more pins are usually required to enable faster performance in the connector product for grounding purposes and for creating more host product operations being done in parallel.
Electrical signals on close spaced conductors may interfere with one another. Capacitive and/or inductive coupling between two adjacent conductors may induce a noise voltage on the neighboring conductor. This unwanted noise voltage is referred to as "cross talk". Controlling and minimizing cross talk is especially important in any high frequency application. In addition, most connector applications contain many interconnection lines. In these cases, cross talk is magnified by the magnitude and number of conductors affected.
By inserting a ground path for the currents to return and hence cause the magnetic field to collapse, cross talk can be minimized. This is a common industry practice. However, even with the presence of a ground return path, electrical field coupling from a driven line to a quiet line typically occurs as a result of the symmetry involved in the connector geometry. Therefore, a tail exit design that simultaneously addresses problems of mechanical density and electrical interference is desirable. It is desirable that a tail exit design address both mechanical density and electrical design characteristics.
High frequency or high speed performance is a function of conductor sizes, materials, geometry, dielectric materials, thickness including air gaps, proximity or relative position or signal conductors to their corresponding ground, and parameters of like kind. In general, the more uniform the above parameters are throughout the entire interconnection path, including the base printed circuit board and connector embodiments, the better the high frequency performance. Cross talk aspects of high speed signaling are described above. Impedance is another important electrical parameter. Both have direct relationships and dependence on the proximity to neighboring conductor elements.
Traditionally, conducting elements are retained within an insulating housing. This is typically performed by placing one or more retention features (typically bumps or barbs) on each edge of a conducting element and forcibly inserting them into a receiving hole or pocket in the insulating housing which is intentionally smaller in size than the corresponding area of a conducting element. A pocket size may be smaller in both dimensions of width and thickness of the cross section or may be just smaller in width in comparison to the bump region of a conducting element. In either case, when a conductive element is forcibly inserted into a housing pocket, the housing is deformed. This deformation occurs since the polymer materials from which a housing is made typically has a strength on the order of 10% of the strength of the copper alloy materials typically used to construct conductive elements. Therefore, deformation in the housing occurs when the ultimate strength of the polymer material used in the insulative housing is exceeded. However, a portion of the housing material typically remains in the elastic region. Thus, elastic equilibrium exists. In addition, polymer materials typically used in the insulative housings are thermoplastics. The modulus of thermoplastics is a function of stress, temperature, and time. The net effect is that there is typically an ongoing and increasing deformation of the geometric shape of the housing pocket over a period of time which is dependent on stresses on the polymer and the temperature of the environment to which it is exposed to. This phenomena is typically referred to as "creep".
Most electrical interconnection products contain more than one conducting path. Typically these have been arranged in longitudinal rows with one or more columns. When an element having symmetrical features is inserted into a housing pocket, the tips of each bump or barb are typically aligned with the bump or barb retention features of neighboring elements. Since a retention feature typically projects from the side of each element, the closest distance between an element and its neighboring elements is typically between opposing retention features. Therefore, a connector housing is thin in this area, and when coupled with stresses induced by an intentional mechanical interference condition, it is possible to initiate an undesired crack through an insulating housing. Such a crack often occurs in a corner region of a pocket due to the stress concentration factors and or in a knit line area. Another problem posed by the close distance between the retention features of a conducting element and the retention features of its neighboring conductor elements is cross talk and impedance. As previously described these phenomena have a direct relationship and dependence on the proximity of neighboring conductor elements.
Thus a conductor or contact retention configuration that increases distance between neighboring conducting elements without sacrificing the density of a connector is desired, thereby reducing electrical and mechanical interference both between the conductor elements and the connector housing.
Traditionally, connector products have contained contacts of like kind throughout, regardless of size or shape. Given this, power has typically been delivered between printed circuit boards and other devices in electronic products by a number of smaller contacts of the same type as that used to pass higher frequency signals. As signal density in connectors increase, the size of conducting elements typically decrease, as does the ability of these elements to transfer electrical power. This is generally due to the electrical conductivity of the contact material and the smaller cross-sectional area. As a result, an increasing number of smaller contacts are required to deliver power, a fact that typically impacts the contact density.
One alternative to the above design is to provide power via a separate power connector with substantial size. Typically these connectors are referred to as "Icons" due to their height and size. Use of these Icon conductors helps alleviate contact density problems, but there is cost associated with placing two types of connectors on one board. In addition, there typically is variation in both horizontal directions, and in the tilt or "Z" direction position between the placement of the Icon and other connectors. Finally, there are typically two mating halves either mounted to another printed circuit board or other housing. This further confounds the positioning variation and typically creates an environment in which connectors mechanically interfere with each other.
Furthermore, as the size and ability of conductor elements to transfer electrical power decreases, problems associated with increased constriction resistance typically increase. In particular, smaller contact geometries may result in contacts that deform or damage more easily, and therefore are more likely to make poor contact with connection points such as solder pads. In addition, smaller contacts are more likely to be overstressed or deformed over time, decreasing contact forces and increasing constriction resistance. When a power contact makes poor connection with a solder pad, either due to misalignment or stress relaxation, heat is typically generated due to increased constriction resistance. As described above, heat generation typically induces further stress relaxation and housing creep. In addition, with power contacts a danger of fire is greater due to the amount of current being transferred through a contact area.
Thus, a power contact configuration capable of resisting deformation, maintaining alignment with solder pad connections, maintaining good electrical contact cross-sectional area and having good rigidity is desired.
To meet demands for smaller, faster, and less expensive products and to address the problems discussed above, improved fine pitched connectors are required. Current connector products do not provide an optimal solution to these opportunities despite the fact that many interconnection schemes have been explored. Therefore, there exists a need for new, high density, high pin count, and low profile electrical connectors that may also provide low cost interconnections.